Special equipment: a potato ricer or food mill; a fine sieve
Put the potatoes in a large pot and add cold water to cover. Bring to a boil over high heat, and cook until a fork inserted into the potatoes meets no resistance, about 30 minutes.
While that¿s going, warm the milk in a small saucepan over low heat. Drain the potatoes well. Working in batches, press the hot potatoes and butter through a potato ricer or a food mill into a large bowl. Stir in the warm milk.
Pass the potatoes through a fine-mesh sieve into a serving bowl. Season with salt and add more butter to taste.